The LC might support resolutions the monitor does not like. VGA at 640 x 480 uses a refresh rate of 60 Hz while the Apple 640 x 480 mode refreshes at 67 Hz. Look at this older post to retrieve specific information to check wether the adapter is made properly (sense pin connection, synchronisation line etc). Just click at the "LAM" tab and find the section "Monitor & Co.".
You might also use the software "activate all resolutions" to enable any resolution the Mac video hardware supports, overwriting the sense pin settings (unfortunately this is only possible when you actually have a working screen).
You might also use the software "activate all resolutions" to enable any resolution the Mac video hardware supports, overwriting the sense pin settings (unfortunately this is only possible when you actually have a working screen).